The present invention relates to a servo technique used for a servo control apparatus incorporated in an optical disc driver.
In devices capable of recording and reproducing information into and from discs, such as CDs (Compact Discs), DVDs (Digital Video Discs), or LDs (Laser Discs), a tracking servo mechanism or focus servo mechanism is required to read out accurately information from information tracks (recorded tracks) of their optical discs. In cases there are obstacles including dark spots, damages or contaminants on the information tracks of the optical disc and a light beam passes across such dark spots, damages or contaminants, a defect occurs in its returned light. Accordingly an input signal obtained from an optical pickup detecting the returned light includes the defect as well. If the defect occurs, a servo error (tracking error (TE) or focus error (FE)) does not indicate its appropriate value during a period of detecting the defect. This inappropriate value of the error signal is known as a xe2x80x9cpseudo error.xe2x80x9d When an actuator is driven based on the pseudo error, a servo operation is disturbed undesirably.
To overcome this situation, a conventional servo control apparatus comprises a device for detecting defects included in the input signal obtained by an optical pickup and a device for preventing the foregoing drawback responsively to a signal detected by a detecting device. More specifically, when a defect is detected, an error signal, which corresponds to an amount displaced from a target value during a normal operation, is replaced by an error signal obtained immediately before the occurrence of a defect (pre-holding) or a low-frequency component of the error signal (integrated signal). This displaced signal is outputted as an alternative servo control signal, with the result that a positional displacement caused by a pseudo error signal (that is, a signal that is not responsible for a displacement from a target value) is lowered.
The defect is found by detecting the fact that the envelope level of an RF signal becomes smaller than a reference level. If irradiated light is scattered by a dark spot, damage, or contaminant on the optical disc, its returned light is reduced in its intensity. As a result, the defect detection causes an inconvenience. Namely, since the envelope level is compared with a certain reference level, the detection that a defect occurs is finally made after the envelope has reduced down to a level smaller than the reference level.
This means that, during an interval starting from the start of the envelope level reduction to the reach of the envelope level to a reference level, a defect will not be detected, even though the defect has already occurred. During such a duration from the occurrence of a defect to the detection thereof (hereafter, called xe2x80x9cdetection delay timexe2x80x9d), a servo control apparatus is obliged to control its servo mechanism based on an error signal affected by the defect. This control urges the optical beam to be moved with an accelerated operation, so that there is a possibility that the optical beam is displaced largely by mistake from a target position.
To reduce such a detection delay time can be realized by increasing the reference level, i.e., raising detection sensitivity of defects. However, raising the detection sensitivity tends to over-detection, which detects even noise which should be ignored as accurate as possible when detecting the signal. The over-detection may bring about interruptions of feedback control conducted based on an error signal originally acquired, which frequently disturbs servo operations. Moreover, because the envelope of the input signal is obtained through LPF processing of the RF signal thereof, a phase shift (phase delay) owing to a phase characteristic of the LPF also results in the detection delay. Influences of the detection delay become larger as a servo band is widened as accuracy required for the servo control is raised.
The present invention has been made with consideration of the foregoing inconveniences that the conventional servo control apparatus has, and an object of the present invention is to provide a servo control apparatus for optical disc drivers, which is able to control servo operations in a stable manner even when defects are detected with delays.
To realize the above object, there is provided a servo control apparatus for an optical disc driver, in which a control signal assigned to a displacement amount of an optical beam from a target value is used to drive an actuator for controlling a position of the optical beam on an optical disc, comprising: a defect detecting device for detecting a defect on the basis of reflected light of the optical beam from the optical disc; a storing device for storing sample data of the control signal obtained during a predetermined interval corresponding to a detection delay of the defect due to at least the defect detecting device on the basis of sample values of the control signal inputted at predetermined sampling intervals; a cancel signal producing device for producing a cancel signal to cancel influences of the control signal obtained during the predetermined interval on the basis of the stored sample data; and an outputting device for outputting the cancel signal as the control signal immediately after the detection of the defect.
According to this configuration, defects occurring on account of dark spots, damages or others on the optical disc are detected from reflected light of an optical beam from the optical disc. Immediately before this detection, sample data of a control signal which are acquired during a predetermined interval of time are memorized. The interval corresponds to a detection delay of the defect-detecting device. Based on the memorized sample data, a cancel signal is produced. The cancel signal is required to cancel influences of the control signal resulted from a pseudo error signal obtained during the above predetermined interval. Immediately after the detection of a defect, the cancel signal is outputted as the control signal. The control signal based on the pseudo error signal causes a force acting on the actuator in an accelerating direction thereof, and the force, which is due to the delay in detecting the defect, disturbs servo operations. The force is however canceled by a force that is produced by the cancel signal and that acts on the actuator in the decelerating direction. Therefore, the optical beam is prevented from shifting from a target position on the optical disc, which leads to servo control with stability.
Preferably, the cancel signal is opposite in polarity to the control signal stored in the storing device as the sample data and is a pulse signal equal to an integrated value of the sample data.
Still preferably, the cancel signal is smaller in a temporal width than the control signal stored as the sample data in the storing device. Accordingly, immediately after the output of a pseudo control signal derived from a pseudo error signal that disturbs servo operations, the cancel signal thus produced can be added instantaneously. Thus a cancel effect against influences of the control signal derived from the pseudo error signal can be enhanced.
It is preferred that the storing device has a FIFO memory and is configured so as to output the sample data using a function of the FIFO memory, and the cancel signal producing device is configured so as to produce the cancel signal by inverting polarities of the outputted sample data.
It is also preferred that the storing device has a FILO memory and is configured so as to output the sample data using a function of the FILO memory, and the cancel signal producing device is configured so as to produce the cancel signal by inverting polarities of the outputted sample data.
Thus, these configurations enable the control signal based on a pseudo error signal to reproduce from the memory as it is. It is therefore possible to easily produce the cancel signal to cancel, with higher precision, influences of the control signal dependent on the pseudo error signal.
In the two configurations with the FIFO and FILO memories, the servo control apparatus may further comprises a reference signal producing device for producing a reference signal by extracting only a lower frequency component of the control signal, wherein the cancel signal producing device is configured so as to invert the polarities of the sample data outputted from the storing device by using as a reference a voltage level of the reference signal produced by the reference signal producing device. This prevents disturbances on account of the inversion of its polarity from occurring.
In all the above configurations, it is preferred that the control signal is an error signal produced from the reflected light. Alternatively, the control signal may be a drive signal to the actuator. Accordingly, the cancel signal for canceling influences of a pseudo error signal, which are brought about by a detection delay of a defect and disturbs servo operations, can be produced with higher precision.